It is known that caprolactone polyols are suitably employed in forming urethane coatings and that they offer a number of advantages over conventional polyester polyols. One of these advantages is that reaction of caprolactone polyols with polyisocyanates generally provides urethane coatings having better weathering characteristics. Clear coatings for exterior application are obtained from caprolactone polyols and aromatic polyisocyanates such as tolylene diisocyanates but such coatings tend to yellow due to the known light instability associated with aromatic diisocyanates.
It is also known that light stable urethane coatings are provided by reaction of caprolactone polyols with aliphatic diisocyanates. However, polyisocyanates of the latter type such as, in particular, 4,4'-methylenebis(cyclohexylisocyanate), either provide toxic coatings due to significant amounts of unreacted isocyanate, or otherwise tend to provide soft coatings not suitable for many high performance applications. One such end-use is the coating of exterior aircraft surfaces. The requirements of such coatings are particularly severe for, in addition to exterior durability characteristics such as high hardness, good impact resistance, adhesion, stain resistance and low temperature flexibility, the coating must possess considerable chemical resistance to potential attack by aircraft hydraulic fluids.
It is, therefore, a primary object of this invention to provide particular caprolactone-based polyols which are especially useful in the formation of high performance coatings.
Another object is to provide such polyols which are used with particular advantage in forming light stable, urethane coatings having good hardness, weatherability and substantial chemical resistance to hydraulic fluids.
A further object is to provide improved urethane coatings for application to external aircraft surfaces.
Various other objects and advantages of this invention will become apparent to those skilled in the art from the accompanying description and disclosure.